Cyclohexene skeletal rearrangement

It was further reported that olefins such as unbranched hexenes (24, 30) undergo only double bond shift without skeletal rearrangement over alumina. On the other hand, rearrangement of the carbon skeleton has been observed in the interconversion of cyclohexene to methylcyclopentenes (14, 15). 

The acid-catalyzed isomerization of cycloalkenes usually involves skeletal rearrangement if strong acids are used. The conditions and the catalysts are very similar to those for the isomerization of acyclic alkenes. Many alkylcyclohexenes undergo reversible isomerization to alkylcyclopentenes. In some cases the isomerization consists of shift of the double bond without ring contraction. Side reactions, in this case, involve hydrogen transfer (disproportionation) to yield cycloalkanes and aromatics. In the presence of activated alumina cyclohexene is converted to a mixture of 1-methyl- and 3-methyl-1-cyclopentene 103... 

Padwa and Gehrlein reported139 that excited-state behavior of 1,6-dihydropyridazines differs from that of other diazacyclohexadienes. Thus photooxidation of 2,5,6-triphenyl-1,6-dihydropyridazine (36a) in 95% ethanol gave pyridazinone (83). However, when irradiation of dihydropyridazines (36) was carried out in cyclohexene in the presence of fumaronitrile, a [2 + 2] cycloadduct (84) was obtained. Furthermore, the authors noted that the dihydropyridazine system underwent a deep-seated skeletal rearrangement when treated with aqueous acid. Thus reaction of 36a with aqueous hydrochloric acid results in the formation of 1,3,4-triphenylpyrazole (85) [Eq. (24)]. 

HTIB [PhI(OH)OTs] has high reactivity toward alkenes and alkynes. Reactions ofHllB with alkenes afford v/c-ditosyloxyalkanes in moderate yield (Scheme 3.73) [225-227]. With cyclohexene and alkyl-substituted alkenes, this reaction proceeds as a stereoselective 5yn-addition, whereas phenyl-substituted alkenes and norbornene give products of skeletal rearrangements. 

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